【作者】 刘玉旺；

【导师】 于明志；

【作者基本信息】 山东建筑大学 ， 热能工程， 2010， 硕士

【摘要】 本文探讨含湿多孔介质导热系数的测试准确性及其影响因素。利用实验测量堆积型多孔介质在不同含湿率时的导热系数,并深入观察和认识孔隙内水分形态、分布及加热时水分演化过程,揭示其与导热系数测量相对误差之间的内在联系。同时在实验基础上,建立了模型描述骨架结构及物理属性对水分分布形式的影响,该模型可用于确定水分形态及分布转变时的临界含湿率。结合骨架与水分形态及分布相互作用和影响的实验及机理分析,确定导热系数测量准确性与多孔介质骨架物理属性、含湿率等的内在关系。利用HotDisk热常数分析仪测量不同粒径堆积型不锈钢珠、玻璃珠和砂子导热系数,分析其导热系数测试结果重复性以及与含湿率之间的关系。结果表明,当不锈钢珠、玻璃珠和砂子含湿率分别低于50%、35%和25%时,含湿介质导热系数较难准确测量；当含湿率大于此值时,含湿介质导热系数容易准确测量。利用高速CCD结合体视显微镜细致观察和分析非饱和含湿情况下试样内部的水分形态及分布并分析其与导热系数测试准确性之间的关系。堆积型不锈钢珠含湿率低于50%、堆积型玻璃珠含湿率低于35%和堆积型砂子含湿率低于25%时,孔隙内水分主要以骨架狭缝处液桥形式存在；此时探头加热过程中,含湿堆积型多孔介质由于水分的蒸发扩散,使得探头周围的试样成分持续发生变化,导致与测试过程中试样构成恒定的假设不符,因此有效导热系数很难准确测量。堆积型不锈钢珠含湿率高于50%、堆积型玻璃珠含湿率高于35%和堆积型砂子含湿率高于25%时,颗粒间水分联通；所以在探头加热过程中,含湿堆积型多孔介质虽然也有水分的蒸发扩散但在毛细回流作用下试样内部较远处水分会及时补充水分减少区域,维持试样的稳定,因此有效导热系数可准确测量。综合实验测量结果和观察现象,本文认为导热系数的测量准确度受孔隙内水分形态及分布的影响。针对非饱和含湿堆积型多孔介质不同试样孔隙内水分状态分界点对应含湿率值不同的现象从孔隙尺度进行分析研究发现,不同试样孔隙内水分呈孤立液桥或相互联通的水团两种不同形态的分界点时对应的含湿率是颗粒表面接触角θ的单值函数；不锈钢和玻璃表面的接触角值分别为72.5°和36.60°,并结合接触角与临界含湿率的关系得到不锈钢珠临界含湿率为48.6%、玻璃珠临界含湿率为35.4%。通过实验与数学模型比较推断,水分在多孔介质骨架表面接触角决定不同含湿率时试样孔隙内水分形态,而水分形态及分布影响试样有效导热系数测量的准确性。接触角越大,孔隙内水分呈孤立液桥时上限含湿率越大,即多孔介质有效导热系数较易准确测量的下限含湿率越大。更多还原

【Abstract】 Thermal conductivity of porous media with different moisture content had been measured, and the accuracy and the influential factors had been studied. Besides, a series of experimental observations were conducted for fully understanding the pore scale water morphology, distribution and evolution in porous media and results revealed that there is some certain inherent relationship between the thermal conductivity measurement relative error and pore water morphologies. A model has been formulated to describe the influence of porous media matrix structure and pore wall thermal properties on pore scale water porphologies. This model can determine the critical transformation moisture content of the water morphology and distribution in pores. By experimental study and theoretical analysis, the influential mechanism of matrix physical properties and moisture content on wet porous media thermal conductivity measurement accuracy has been revealed.Thermal conductivity of piled stainless steel beads, glass beads and sands with different particle size had been measured by a Hot Disk constant analyzer. The repeatability and relative error of the measured values and also their variation versue the moisture content had been analysed. The results showed that the thermal conductivities of samples with relatively high water content are easy to be measured correctively but very difficult for the stainless steel beads, glass beads and sands with the moisture content less than 50%, 35% and 25% respectively.Experimental investigations were also conducted to visually observe water morphology, distribution and evolution performance in pores respectively by a CCD combined with a microscope and their affection to the test results of the thermal conductivity measuring based on these experiments were analysed. Most of the water exists as seperated liquid bridges among particles in the porous media when moisture content is less than 50%，35% and 25% for cumulate stainless steel beads, silver beads and sands respectively. Due to the sensor heating, water evaporates and transport processes diffuse in the pores near the sensor. As a result, the properties of this region become different from their originals. So the measured thermal conductivities are not the real value of the wet sample in its original state. As contrast, the water mostly exists as water masses in the porous media when moisture content is higher than the value, though the water evaporates near the heating resource, the near heating source pores are complemented as soon as the water driven back from the relatively far pores by the capillary force. So the correct thermal conductivity can be measured easily. According to the experimental results and phenomena, we can consider that morphology and distribution of pore water has effected the measurement accuracy of thermal conductivity.The reason why different porous media have different critical moisture content for thermal conductivity can be measured correctively has also been studied in this thesis. The critical moisture content is a single-value function of the water contact angle on particle surface. The contact angles of water on the stainless steel and glass surface are 72.5°and 36.6°respectively, so the largest moisture contents for water mostly exist as isolated liquid bridges are 48.6% and 35.4% indicated by the model formulated in this thesis. Comparison between experimental results and mathematical model showes that the water contact angle on porous media matrix surface determines water morphology and distribution in pores at certain moisture content, and water morphology and distribution then affect measurement accuracy of thermal conductivity. The larger the contact angle is, the higher upper water content limit that the water exists as isolated liquid bridges is, i.e. the higher the lower water content limit that the wet porous media can be easily measured correctly is.更多还原